Schottky barrier field effect transistor large in withstanding voltage and small in distortion and return-loss

ABSTRACT

A Schottky barrier field effect transistor has a gate electrode formed with a field plate in order to achieve a high withstanding voltage, wherein the thickness of the dielectric layer between the channel layer and the field plate, the distance between the Schottky contact and the drain and the length of the field plate are to be fallen within the range between 300 nanometers thick to 600 nanometers thick, the range from 800 nanometers long to 3000 nanometers long and the range between the distance between the Schottky contact and the drain plus minus 400 nanometers, respectively, so that the distortion and the return-loss are improved without sacrifice of the withstanding voltage.

FIELD OF THE INVENTION

[0001] This invention relates to a Schottky barrier field effect transistor and, more particularly, to a high-frequency large-output Schottky barrier field effect transistor.

DESCRIPTION OF THE RELATED ART

[0002] Gallium arsenide, i.e., GaAs has large electron mobility, which is five to six times larger than the electron mobility of silicon. The peak value of the saturated drift velocity in gallium arsenide is twice as large as that in the silicon. Another attractive feature of gallium arsenide is that a semi-insulating substrate is obtained. These features are put to practical use in the field of semiconductor device manufacturing. The conventional field effect transistor fabricated on a silicon substrate hardly responds to a microwave signal. However, the gallium arsenide Schottky barrier field effect transistor is well responsive to the microwave signal, and is employed in microwave electronic devices. In fact, the gallium arsenide Schottky barrier field effect transistor tends to supersede the electron tubes. Demand for a high-frequency large-output Schottky barrier field effect transistor is getting stronger and stronger.

[0003] A typical example of the gallium arsenide Schottky barrier field effect transistor is fabricated on a semi-insulating gallium arsenide substrate, and the semi-insulating gallium arsenide substrate is overlain by a thin gallium arsenide channel layer. A source electrode and a drain electrode are held in ohmic contact with the thin gallium arsenide channel layer, and are spaced from each other. A gate electrode is formed on the thin gallium arsenide channel layer between the source electrode and the drain electrode, and the gate electrode and the thin channel layer form a Schottky barrier therebetween. A depletion layer extends from the Schottky barrier into the thin gallium arsenide channel layer.

[0004] A problem inherent in the prior art gallium arsenide Schottky barrier field effect transistor is destruction due to a reverse bias voltage applied between the gate electrode and the drain electrode. When the reverse voltage is applied between the gate electrode and the drain electrode, the electric field is concentrated around the edge of the gate electrode on the drain side, and the destruction takes place. In order to prevent the gallium arsenide Schottky barrier field effect transistor from the destruction, a field plate is formed in the gate electrode, and dielectric layers of silicon dioxide is provided beneath the side portions of the gate electrode as disclosed in Japanese Patent Application laid-open Nos. 63-87773 and 2000-100831.

[0005]FIG. 1 shows a cross section of the prior art gallium arsenide Schottky barrier field effect transistor disclosed in Japanese Patent Application laid-open No. 2000-100831. The prior art gallium arsenide Schottky barrier field effect transistor is fabricated on a gallium arsenide semi-insulating substrate 61. The gallium arsenide semi-insulating substrate 61 is overlain by a channel layer 62. Contact layers 63 are formed on the upper surface of the channel layer 62, and are spaced from each other so that a part of the channel layer 62 is exposed to the recess between the contact layers 63. A source electrode 67 and a drain electrode 68 are respectively formed on the contact layers 63, and are held in ohmic contact therewith. The contact layers 63 and the exposed surface of the channel layer 62 are covered with a dielectric layer 64, and a gate electrode 65 is held in contact with the channel layer 62 through a contact hole formed in the dielectric layer 64. The gate electrode 65 has a field plate 69. The field plate 69 projects from the remaining portion of the gate electrode 65 toward the drain electrode 68, and is opposed through the dielectric layer 64 to the channel layer 62.

[0006] The laid-open teaches that the dielectric layer 64 is improved in withstanding voltage when the following conditional expressions are satisfied.

1<ε<5  (1)

25<t/ε<70  (2)

[0007] where ε is dielectric constant of the dielectric layer 64 and t is the thickness of the dielectric layer 64.

[0008] The dielectric layer 64 is assumed to be formed of SiO₂. The dielectric constant of silicon dioxide is of the order of 3.9. Then, the preferable thickness t is calculated from the conditional expressions (1) and (2), and is to be fallen within the range greater than 97.5 nanometers and less than 273 nanometers, i.e., 97.5 nm<t<273 nm.

[0009] There is found description that the dielectric layer 64 is to be 200 nanometers thick in the prior art Schottky barrier field effect transistor in order to exhibit good withstanding voltage characteristics. However, the prior art documents are silent to the contour of the field plate.

[0010] Using a microwave wide band amplifier, the present inventors measured the return-loss representative of the reduction in the gain of the amplifier. The microwave wide band amplifier formed a part of a communication system, and included two Schottky barrier field effect transistors different in thickness of the dielectric layer from each other. The return loss was measured at 800 MHz. In case where the dielectric layer was 200 nanometers thick, the return-loss was −15 dB. On the other hand, when the dielectric layer was 400 nanometers thick, the return-loss was −18 dB.

[0011] The field plate 69, the channel layer 62 and the dielectric layer 64 form in combination a capacitor, and the parasitic capacitance is inversely proportional to the thickness of the dielectric layer 64. When the parasitic capacitance is increased, the gain is decreased due to the Miller effect. On the other hand, if the dielectric layer is decreased to a certain thickness greater than 200 nanometers, the suppression against the electric field concentration is weakened, and, accordingly, the withstanding voltage becomes low. The suppression against the electric field concentration further results in strong distortion under the reception of excessively large input power. Namely, when the radio-frequency input signal has an excessively large power, the electric charge is accumulated at the boundary between the channel layer and the dielectric layer on the drain side, and causes the output signal to have wide distortion. When the prior art Schottky barrier field effect transistor is incorporated in a wide-amplitude high-power electric circuit, the problem is serious.

SUMMARY OF THE INVENTION

[0012] It is therefore an important object of the present invention to provide a Schottky barrier field effect transistor, which is small in return-loss, high in withstanding voltage and small in distortion.

[0013] In accordance with one aspect of the present invention, there is provided a Schottky barrier field effect transistor fabricated on a substrate comprising a channel layer formed of a first compound semiconductor, a source structure connected to a first portion of the channel layer, a drain structure connected to a second portion of the channel layer and spaced from the source structure for forming a recess, a dielectric layer covering the channel layer, a part of the source structure and a part of the drain structure and having a contact hole between the source structure and the drain structure and a gate electrode having a Schottky contact portion held in contact with the channel layer through the contact hole for producing a Schottky contact with the channel layer and a field plate extending from the Schottky contact portion toward the drain structure on the dielectric layer, and a thickness of the dielectric layer between the channel layer and the field plate is equivalent to a thickness of silicon dioxide layer equal to or greater than 300 nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The features and advantages of the Schottky barrier field effect transistor will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which:

[0015]FIG. 1 is a cross sectional view showing the structure of the prior art Schottky barrier field effect transistor;

[0016]FIG. 2 is a cross sectional view showing the structure of a Schottky barrier field effect transistor according to the present invention;

[0017]FIGS. 3A to 3E are cross sectional views showing a process sequence for fabricating the Schottky barrier field effect transistor according to the present invention;

[0018]FIG. 4 is a graph showing a relation between a normalized capacitance and a gate/drain overlay distance;

[0019]FIG. 5 is a graph showing a relation between a distortion and an input signal level;

[0020]FIG. 6 is a graph showing a relation between a return-loss and the combination of the thickness of a dielectric layer and the gate/drain overlay distance;

[0021]FIG. 7A is a cross sectional view showing the structure of another Schottky barrier field effect transistor according to the present invention; and

[0022]FIG. 7B is a plane view showing boundaries among the layers incorporated in the Schottky barrier field effect transistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] First Embodiment

[0024] Referring to FIG. 2 of the drawings, a Schottky barrier field effect transistor embodying the present invention is fabrication on a semi-insulating substrate of gallium arsenide. A channel layer 1 is laminated on the major surface of the semi-insulating substrate, and is formed of n-type gallium arsenide. A drain contact layer 3 and a source contact layer 4 are formed on the channel layer 1, and the drain contact layer 3 is spaced from the source contact layer 4. The drain contact layer 3 and the source contact layer 4 are formed of n-type gallium arsenide. A part of the channel layer 1 is exposed to the gap or recess between the drain contact layer 3 and the source contact layer 4. The drain contact layer 3, the part of the channel layer 1 and the source contact layer 4 are covered with a dielectric layer 5. Though not shown in FIG. 2, a drain electrode and a source electrode are held in contact with the drain contact layer 3 and the source contact layer 4 through contact holes formed in the dielectric layer 5.

[0025] A contact hole is formed in the dielectric layer 5, and reaches the part of the channel layer 1 between the drain contact layer 3 and the source contact layer 4. A gate electrode 7 is formed on the dielectric layer 5, and is held in contact with the channel layer 1 through the contact hole. The gate electrode 7 is formed of metal, and Schottky barrier takes place between the channel layer 1 and the gate electrode 7. The gate electrode 7 has a field plate 11, and the field plate 11 projects from the remaining portion of the gate electrode 7 toward the drain contact layer 3. The remaining portion passes through the contact hole, and reaches the channel layer 1. The part of the gate electrode 7 in the contact hole is hereinbelow referred to as “Schottky contact portion”, and the Schottky contact portion is labeled with 7 a.

[0026] The Schottky contact portion 7 a is spaced from the drain contact layer 3, and reference “Lrgd” is indicative of the distance between the contact portion 7 a and the drain contact layer 3. On the other hand, the distance between the Schottky contact portion 7 a and the leading end of the field plate 11 is referred to as “gate/drain overlay distance”, and the gate/drain overlay distance is designated by reference “Lgd”. In this instance, the distance Lrgd is fallen within the range between 800 nanometers and 3000 nanometers, and the gate/drain overlay distance Lgd is designed to be “Lrgd plus minus 400 nanometers”, i.e., (Lrgd ±400 nanometers).

[0027] Description is hereinbelow made on a process for fabricating the Schottky barrier field effect transistor with reference to FIGS. 3A to 3E. First, the semi-insulating substrate 10 of gallium arsenide is prepared. The semi-insulating substrate 10 is placed in the chamber of a molecular beam epitaxial system (not shown). N-type gallium arsenide doped with silicon at 2×10¹⁷ cm⁻³ is epitaxially grown to 25 nanometers thick, and n-type gallium arsenide doped with silicon at 5×10⁷ cm⁻³ to 150 nanometers thick. The n-type gallium arsenide doped with silicon at 2×10¹⁷ cm⁻³ forms the channel layer 1, and the n-type gallium arsenide doped with silicon at 5×10^(17 cm) ⁻³ forms an n-type gallium arsenide layer 2 as shown in FIG. 3A.

[0028] Photo-resist solution is spun onto the n-type gallium arsenide layer 2, and is baked so as to form a photo-resist layer (not shown). A pattern image for the source/drain contact layers 3/4 is transferred from a photo mask (not shown) to the photo-resist layer, and a latent image is formed in the photo-resist layer. The pattern image is developed, and the photo-resist layer is patterned into a photo-resist etching mask (not shown). Thus, the photo-resist etching mask is formed through photo-lithographic techniques.

[0029] Using the photo-resist etching mask, the n-type gallium arsenide layer 2 is selectively removed in gaseous etchant containing chlorine. A recess is formed in the n-type gallium arsenide layer 2, and, accordingly, the n-type gallium arsenide layer 2 is separated into the drain contact layer 3 and the source contact layer 4. The photo-resist etching mask is stripped off.

[0030] Subsequently, silicon dioxide is deposited to 400 nanometers thick over the entire surface of the resultant structure by using a chemical vapor deposition, and forms the dielectric layer 5. The resultant structure in this stage is shown in FIG. 3B.

[0031] A photo-resist etching mask (not shown) is formed on the dielectric layer 5 by using the photo-lithographic techniques, and has a pattern image for the contact hole. Using the photo-resist etching mask, the dielectric layer 5 is selectively removed in gaseous etchant containing CHF₃ or SF₆. Thus, the contact hole for the gate electrode 7 is formed in the dielectric layer 5, and a part of the channel layer 1 is exposed to the contact hole. The photo-resist etching mask is stripped off.

[0032] Using the dielectric layer 5 as an etching mask, the channel layer 1 is partially etched, and the channel layer 1 is formed with a shallow recess as shown in FIG. 3C. The shallow recess is of the order of 5 nanometers deep.

[0033] Subsequently, WSi is deposited to 200 nanometers thick over the entire surface of the resultant structure by using a sputtering technique, and Au is further deposited to 400 nanometers thick over the WSi layer also by using the sputtering technique. The WSi and Au form a metallic layer 6. The metallic layer 6 penetrates into the shallow recess through the contact hole, and is held in contact with the channel layer 1 as shown in FIG. 3D.

[0034] A photo-resist etching mask (not shown) is formed on the metallic layer 6 by using the photo-lithographic techniques, and the metallic layer 6 is patterned into the gate electrode 7 through an ion milling. The gate electrode 7 is on a predetermined area of the dielectric layer 5, and the dielectric layer 5 is exposed on both sides of the gate electrode 7. The photo-resist etching mask is stripped off. The resultant structure is shown in FIG. 3E.

[0035] A photo-resist etching mask (not shown) is formed on the dielectric layer 5 by using the photo-lithographic techniques, and has a pattern image for the drain/ source contact holes. Using the photo-resist etching mask, the dielectric layer 5 is selectively etched so that a drain contact hole (not shown) and a source contact hole (not shown) are formed in the dielectric layer 5. The drain contact layer 3 is exposed to the drain contact hole, and the source contact layer 4 is exposed to the source contact hole. Ni, AuGe and Au are successively deposited to 8 nanometers thick, 50 nanometers thick and 250 nanometers thick by using a vacuum evaporation, and are formed into a drain electrode and a source electrode (not shown).

[0036] The present inventors fabricated samples of the Schottky barrier field effect transistor through the process sequence described hereinbefore, and investigated characteristics of the Schottky barrier field effect transistor. First, the present inventors prepared two groups of samples. The samples of the first group respectively had the dielectric layers 5 of 200 nanometers thick, and were different in gate/ drain overlay distance Lgd from one another. On the other hand, the samples of the second group respectively had the dielectric layers 5 of 400 nanometers thick, and were different in gate/ drain overlay distance Lgd from one another. The channel layer 1, the dielectric layer 5 and the gate electrode 7 formed a capacitor. The normalized capacitance was calculated on the n-type gallium arsenide layers on the assumption that the capacitor was of the parallel plate type. The influence of the resistance ratio and the fringing effect on the capacitance was taken into account. The normalized capacitance was representative of the parasitic capacitance coupled to the Schottky barrier field effect transistor. The relation between the normalized capacitance and the parasitic capacitance was expressed as

(parasitic capacitance)=ε0×ε×(normalized capacitance)  (3)

[0037] where ε0 was the dielectric constant in vacuum and ε was the dielectric constant of the dielectric layer 5. The dielectric constant in vacuum was 8.854×10⁻⁶ [pF/μm], and the dielectric constant of silicon dioxide was 3.9. The unit of the parasitic capacitance was pF.

[0038] Using the samples in the first group, the present inventors calculated the normalized capacitance, and plotted the values of the normalized capacitance as indicated by real line PL1 in FIG. 4. Similarly, the present inventors calculated the normalized capacitance of the samples in the second group, and plotted the values of the normalized capacitance as indicated by broken line PL2 in FIG. 4. Comparing plots PL1 with plots PL2, the ratio of the normalized capacitance at 200 nanometers thick to the normalized capacitance at 400 nanometers thick was decreased inversely to the gate/ drain overlay distance Lgd. In the samples where the gate/drain overlay distance Lgd was 400 nanometers long, the normalized capacitance at 200 nanometers thick was only 1.2 times larger than the normalized capacitance at 400 nanometers thick. When the gate/drain overlay distance was increased to 1200 nanometers long, the normalized capacitance at 200 nanometers thick was 1.8 times larger than the normalized capacitance at 400 nanometers thick. In other words, the samples in the first group were more sensitive to the gate/drain overlay distance than the samples in the second group.

[0039] Subsequently, the present inventors prepared samples different in gate/drain overlay distance Lgd. The distance Lrgd of the samples was 1100 nanometers long. The first sample had the gate/drain overlay distance Lgd of 400 nanometers long, and the second sample had the gate/drain overlay distance Lgd of 1300 nanometers long.

[0040] The present inventors assembled the samples into a microwave communication wide band amplifier, and measured the distortion in terms of the input signal level. The input signal level was varied from −5 dBV to +1 dBV, and the distortion was plotted as shown in FIG. 5. Plots PL3 and plots PL4 stood for the sample at 400 nanometers long and the sample at 1300 nanometers long. Although the sample at 400 nanometers long rapidly increased the distortion around the input signal level of +5 dBV, the distortion of the sample at 1300 nanometers long was less dependent on the input signal level. In fact, when the input signal level was increased from −5 dBV to +10 dBV, the sample at 400 nanometers long increased the distortion by 3.5 dB. However, the distortion of the sample at 1300 nanometers long was gently decreased toward the input signal level of +10 dBV. Thus, the long gate/drain overlay distance was effective against the distortion.

[0041] Subsequently, the present inventors fabricated samples of the Schottky barrier field effect transistor different in both of the thickness and the gate/drain overlay distance Lgd. The first sample had the dielectric layer 5 of 200 nanometers thick and the gate/drain overlay distance Lgd of 1300 nanometers long. The second sample had the dielectric layer 5 of 400 nanometers thick and the gate/drain overlay distance Lgd of 1300 nanometers long. The third sample had the dielectric layer 5 of 400 nanometers thick and the gate/drain overlay distance Lgd of 400 nanometers long. Thus, first sample a was equal in the gate/drain overlay distance to the samples of the second sample, but was different in the thickness of the dielectric layer from the second sample. On the other hand, the second sample was equal in thickness of the dielectric layer 5 to the third sample, but was different in the gate/drain overlay distance from the second sample.

[0042] The present inventors calculated the normalized capacitance of the samples, and measured the return-loss. The present inventors plotted the values of the return-loss in terms of the normalized capacitance as shown in FIG. 6. Comparing plot a with plot b, the increase in the thickness from 200 nanometers thick to 400 nanometers thick resulted in that the return loss was decreased by 2 dB. On the other hand, when the gate/drain overlay distance was decreased from 1300 nanometers long to 400 nanometers long, i.e., at 4/13, the return-loss was decreased by 1 dB.

[0043] Thus, the present inventor discovered that both of the thickness of the dielectric layer 5 and the gate/drain overlay distance Lgd were influential on the withstanding voltage, distortion and return-loss. The present inventors fabricated samples of the Schottky barrier field effect transistor which were different in the combination of the thickness of the dielectric layer 5 and the gate/drain overlay distance as similar to the samples used for the evaluation shown in FIG. 6. The samples had the distance Lrgd from 800 nanometers long to 3000 nanometers long. The distance Lrgd was determined on the basis of the dimensions of actual Schottky barrier field effect transistors to be required for the microwave wide band amplifier.

[0044] The present inventors evaluated the samples in order to determine the range of the thickness and the range of the gate/drain overlay distance Lgd. The present inventors measured the withstanding voltage, the distortion and the return-loss for each of the samples. Even though the thickness of the dielectric layer 5 was out of the range expressed by the conditional expression (2), i.e., 97.5 nm<t<273 nm in case of silicon dioxide, the distortion and the return-loss were acceptable without sacrifice of the withstanding voltage in so far as the following conditions were satisfied. The first condition was that the dielectric layer 5 was fallen within the range from 300 nanometers thick to 600 nanometers thick, and the second condition was that the gate/drain overlay distance Lgd was Lrgd ±400 nanometers.

[0045] Even if the gate/drain overlay distance Lgd was long, the thin dielectric layer 5 makes the relief to the electric field concentration strong. The electrostatic capacitance per unit area of the laminated structure of the field plate 11, the channel layer 1 and the dielectric layer 5 is effective against the electric field concentration on the drain side rather than on the remaining portion of the gate electrode 7, and makes the dispersion of the strength of the electric field close to the ideal dispersion. This results in the relief of the electric field concentration. The electrostatic capacitance makes the electric lines of force extending from the ionized donor terminated thereat. This results in that, even if the excess radio frequency signal is inputted, the electrostatic capacitance suppresses the accumulation of the electric charge at the boundary of the semiconductor layer on the drain side. For this reason, the distortion is made small.

[0046] As will be understood from the foregoing description, the structure in the recess designed in accordance with the present invention permits the Schottky barrier field effect transistor to achieve good transistor characteristics, i.e., high withstanding voltage, small distortion and small return-loss.

[0047] Second Embodiment

[0048] Turning to FIG. 7A, another Schottky barrier field effect transistor embodying the present invention is fabricated on a semi-insulating substrate 10. The Schottky barrier field effect transistor implementing the second embodiment comprises a channel layer 1, a drain contact layer 3, a source contact layer 4, a dielectric layer 5 and a gate electrode 7.

[0049] The channel layer 1 is laminated on the major surface of the semi-insulating substrate 10, and is formed of n-type gallium arsenide, the drain contact layer 3 and the source contact layer 4 are formed on the channel layer 1, and the drain contact layer 3 is spaced from the source contact layer 4. The drain contact layer 3 and the source contact layer 4 are formed of n-type gallium arsenide. A part of the channel layer 1 is exposed to the gap or recess between the drain contact layer 3 and the source contact layer 4. The drain contact layer 3, the part of the channel layer 1 and the source contact layer 4 are covered with the dielectric layer 5. Though not shown in FIG. 7A, a drain electrode and a source electrode are held in contact with the drain contact layer 3 and the source contact layer 4 through contact holes formed in the dielectric layer 5.

[0050] A contact hole is formed in the dielectric layer 5, and reaches the part of the channel layer 1 between the drain contact layer 3 and the source contact layer 4. The gate electrode 7 is formed on the dielectric layer 5, and is held in contact with the channel layer 1 through the contact hole. The gate electrode 7 is formed of metal, and a Schottky barrier takes place between the channel layer 1 and the gate electrode 7. The gate electrode 7 has a field plate 11, and the field plate 11 projects from the remaining portion of the gate electrode 7 toward the drain contact layer 3. The remaining portion passes through the contact hole, and reaches the channel layer 1. The gate electrode 7 has a Schottky contact portion 7 a on the dielectric layer 5.

[0051] The Schottky contact portion 7 a is spaced from the drain contact layer 3 by the distance Lrgd. In this instance, the distance Lrgd is fallen within the range between 800 nanometers and 3000 nanometers, and the gate/drain overlay distance Lgd is designed to be (Lrgd ±400 nanometers).

[0052] The source contact layer 4 is held in contact with the channel layer 1, and the inner edge of the source contact layer 4 is labeled with “a” in FIG. 7B. Similarly, the gate electrode 7 is held in contact with the dielectric layer 5, and the left edge of the gate electrode 7 extends along line “b”. The dielectric layer 5 is held in contact with the channel layer 1, and has the left inner surface along line “c” and the right inner surface along line “d”. The field plate 11 is held in contact with the dielectric layer 5, and is formed into a comb-like configuration. For this reason, a zig-zag line “e” is indicative of the end surface of the field plate 11. The drain contact layer 3 is held in contact with the channel layer 1, and has the inner end indicated by line “f”.

[0053] The Schottky barrier field effect transistor is fabricated through a process similar to that of the first embodiment except that the photo-resist etching mask for the gate electrode has the edge surface extending along line “e”.

[0054] The comb-like field plate 11 causes the contact area between the gate electrode 7 and the dielectric layer 5 and, accordingly, the area opposed to the channel layer 1 to decrease on the drain side. The gate electrode 7, the dielectric layer 5 and the channel layer 1 form a capacitor as similar to those of the first embodiment. The electrostatic capacitance C of the capacitor is expressed as

C=εS/d  (4)

[0055] where ε is the dielectric constant of the dielectric layer 5, S is the area opposed to the channel layer 1 and d is the distance between the gate electrode 7 and the channel layer 1. Since the area S is narrowed due to the comb-like end surface of the field plate 11, the capacitance C is decreased on the drain side. Thus, the parasitic capacitance is reduced by virtue of the comb-like end surface.

[0056] The present inventors fabricated a sample of the Schottky barrier field effect transistor implementing the second embodiment, and evaluated the sample. The present inventors assembled the sample in a microwave wide band amplifier, and measured the return-loss. The present inventors further fabricated a comparative sample which did not have the comb-like end surface, i.e., had a flat end surface, and assembled the comparative sample in the microwave wide band amplifier for measuring the return-loss. The present inventors confirmed that the return-loss of the sample was smaller the return-loss of the comparative sample by 0.5 dB.

[0057] The present inventors evaluated the Schottky barrier field effect transistor implementing the second embodiment through the experiments similar to the Schottky barrier field effect transistor implementing the first embodiment. The samples of the Schottky barrier field effect transistor implementing the second embodiment achieved small distortion and small return-loss without sacrifice of the withstanding voltage in the range described in conjunction with the first embodiment.

[0058] In the above-described embodiments, the drain contact layer 3 and the drain electrode (not shown) as a whole constitute a drain structure, and the source contact layer 4 and the source electrode (not show) form in combination a source structure.

[0059] Although particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.

[0060] In the process described hereinbefore, the compound semiconductors are grown by using the molecular beam epitaxial system. However, the compound semiconductors may be grown by using a metal organic molecular beam epitaxial system. A multiple-recess structure may be employed in another Schottky barrier field effect transistor according to the present invention.

[0061] The compound semiconductors may be different from the gallium arsenide.

[0062] The dielectric layer 5 may be formed of an insulating substance different from silicon dioxide. In this instance, if the dielectric layer is equivalent to the silicon dioxide layer of 300 nanometers thick to 600 nanometers thick, the above-described advantages are achieved. 

What is claimed is:
 1. A Schottky barrier field effect transistor fabricated on a substrate, comprising: a channel layer formed of a first compound semiconductor; a source structure connected to a first portion of said channel layer; a drain structure connected to a second portion of said channel layer, and spaced from said source structure for forming a recess; a dielectric layer covering said channel layer, a part of said source structure and a part of said drain structure, and having a contact hole between said source structure and said drain structure; and a gate electrode having a Schottky contact portion held in contact with said channel layer through said contact hole for producing a Schottky contact with said channel layer and a field plate extending from said Schottky contact portion toward said drain structure on said dielectric layer, a thickness of said dielectric layer between said channel layer and said field plate being equivalent to a thickness of silicon dioxide layer equal to or greater than 300 nanometers.
 2. The Schottky barrier field effect transistor as set forth in claim 1, in which a distance between said Schottky contact portion and said drain structure ranges from 800 nanometers to 3000 nanometers, and said field plate has a length expressed as Lrgd ±400 nanometers where Lrgd is said distance between said Schottky contact portion and said drain structure.
 3. The Schottky barrier field effect transistor as set forth in claim 2, in which said drain structure includes a drain contact layer formed of a second compound semiconductor, and said distance is measured from said Schottky contact portion and said drain contact layer.
 4. The Schottky barrier field effect transistor as set forth in claim 3, in which said first compound semiconductor and said second compound semiconductor are n-type gallium arsenide different in dopant concentration.
 5. The Schottky barrier field effect transistor as set forth in claim 1, in which said thickness of said dielectric layer is equal to or less than 600 nanometers, and said field plate has a length expressed as Lrgd ±400 nanometers where Lrgd is a distance between said Schottky contact portion and said drain structure.
 6. The Schottky barrier field effect transistor as set forth in claim 5, in which said drain structure includes a drain contact layer formed of a second compound semiconductor, and said distance is measured from said Schottky contact portion and said drain contact layer.
 7. The Schottky barrier field effect transistor as set forth in claim 1, in which said dielectric layer is formed of silicon dioxide, and said thickness of said dielectric layer ranges from 300 nanometers to 600 nanometers.
 8. The Schottky barrier field effect transistor as set forth in claim 7, said field plate having a length expressed as Lrgd ±400 nanometers where Lrgd is a distance between said Schottky contact portion and said drain structure.
 9. The Schottky barrier field effect transistor as set forth in claim 8, in which said distance is measured between said Schottky contact portion and a drain contact portion held in contact with said second portion of said channel layer and formed of a second compound semiconductor.
 10. The Schottky barrier field effect transistor as set forth in claim 9, in which said first compound semiconductor and said second compound semiconductor are n-type gallium arsenide different in dopant concentration.
 11. The Schottky barrier field effect transistor as set forth in claim 1, in which said field plate had a leading end portion with a comb-like configuration.
 12. The Schottky barrier field effect transistor as set forth in claim 11, a distance between said Schottky contact portion and said drain structure ranging from 800 nanometers to 3000 nanometers, and said field plate having a length expressed as Lrgd ±400 nanometers where Lrgd is said distance between said Schottky contact portion and said drain structure.
 13. The Schottky barrier field effect transistor as set forth in claim 12, in which said drain structure includes a drain contact layer formed of a second compound semiconductor, and said distance is measured from said Schottky contact portion and said drain contact layer.
 14. The Schottky barrier field effect transistor as set forth in claim 13, in which said first compound semiconductor and said second compound semiconductor are n-type gallium arsenide different in dopant concentration.
 15. A Schottky barrier field effect transistor fabricated on a substrate, comprising: a channel layer formed of a first compound semiconductor; a source structure connected to a first portion of said channel layer; a drain structure connected to a second portion of said channel layer, and spaced from said source structure for forming a recess; a dielectric layer covering said channel layer, a part of said source structure and a part of said drain structure, and having a contact hole between said source structure and said drain structure; and a gate electrode having a Schottky contact portion held in contact with said channel layer through said contact hole for producing a Schottky contact with said channel layer and a field plate extending from said Schottky contact portion toward said drain structure on said dielectric layer, a thickness of said dielectric layer between said channel layer and said field plate being equivalent to a thickness of silicon dioxide layer equal to or greater than 300 nanometers, said field plate having a length expressed as (Lrgd ±400 nanometers) where Lrgd is a distance between said Schottky contact portion and said drain structure.
 16. The Schottky barrier field effect transistor as set forth in claim 15, in which said drain structure includes a drain contact layer formed of a second compound semiconductor, and said distance is measured from said Schottky contact portion and said drain contact layer.
 17. The Schottky barrier field effect transistor as set forth in claim 16, in which said first compound semiconductor and said second compound semiconductor are n-type gallium arsenide different in dopant concentration. 